1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic apparatus and a laser printer and, more particularly, to an improvement of a charger in an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus such as an electrophotographic apparatus or laser printer, a corona discharging system has been widely used as a charger for uniformly performing a charging operation on an image bearing member, because the charger has a simple structure and high charging efficiency.
Corona discharging schemes are classified into a corotron in which a high voltage is applied to a corona wire electrode accommodated in a shield case to supply corona ions to an image bearing member from an opening opposite the image bearing member, a scorotron in which a screen-like grid is arranged in an opening opposite to an image bearing member and a predetermined voltage is applied to the grid to control corona ions supplied to the image bearing member.
A laser printer generally employs a scheme in which a negative charge type organic photoconductor (to be referred to as an OPC hereinafter) is used as an image bearing member to generate an image by inversion. Since the OPC must have a charger for discharging negative corona ions having unstable discharging characteristics, a scorotron scheme having excellent stability is conventionally used.
When a charger of this scorotron scheme is used, the charging ability of the charger is structurally determined in consideration of a moving time of an image carrier, the charging characteristics of the image bearing member, and a surface potential of the image bearing member immediately before a charging operation.
In recent years, as the size of an image forming apparatus has decreased, high-speed operation of the image forming apparatus is required by the market. Under these circumstances, the size of a charger must be decreased to accommodate the charger in a limited space. In addition, since a charging time is decreased to shorten a moving time of an image bearing member, a charger must have a high charging ability.
A surface potential of the image bearing member immediately before the charging operation, which potential influences a charging operation, tends to decrease the charging capacity of the charger in an image forming system. That is, a separating means located next to a transfer unit has been changed from a mechanical separating apparatus using a belt or grippers to a corona separating apparatus having a shield case and a corona wire in consideration of installation space and cost. That is, in a discharging separation scheme, a high negative voltage is applied to an electrode having a shape edge using a small curvature obtained by decreasing the diameter of the image bearing member, and corona discharge is generated by an electric field between the electrode having the sharp edge and a transfer medium having transfer charges. Although transfer charges of a transfer medium and an image bearing member ar sufficiently destaticized in this scheme, since the scheme requires a discharging separation transformer, the cost is disadvantageously increased.
In order to keep the cost low, the following scheme is practically used. That is, a discharging lamp or a grounded electrode having a sharp edge is arranged immediately before a transfer unit to weaken an electrostatic attraction force between a transfer medium and an image bearing member, thereby performing a separating operation. In this scheme, it is an object to discharge the transfer medium, in contrast to the above scheme in which the high negative voltage is applied to positively remove positive charges received by a transfer corona apparatus over the entire areas of the transfer medium and the image bearing member. For this reason, this scheme does not have an ability of sufficiently discharging the entire area of the image bearing member. Therefore, after transferring and separating operations, positive charges remaining on the surface of the image bearing member are left through cleaning and preexposing steps, and a discharging operation is not performed in these steps. A charging step is initiated in this state. In the cleaning and preexposing steps, when a negative OPC is used as the image bearing member, only a destaticizing operation of negative charges is performed, but a discharging effect for positive charges cannot be expected.
Therefore, when a charger does not have a sufficiently high negative charge having a polarity opposite to the positive charge of the image bearing member immediately before the charging step, a charging potential is lowered as indicated by V.sub.0 ' in FIG. 1E of electrostatic processes shown in FIGS. 1A to 1F, and a fog t' causing background scumming occurs due to a potential difference between development biases V.sub.B and V.sub.0 ' in a developing operation as shown in FIG. 1F. This phenomenon will be described with reference to FIGS. 2A to 2B.
FIGS. 2A and 2B are views showing an image bearing member viewed from a transfer unit side. A region indicated by hatched lines in FIG. 2A is a region of the image bearing member which directly receives positive charges serving as transfer corona ions. A region between transfer media Pl and P2 and regions on both the sides of the image bearing member except for the transfer media receive positive charges. When these regions are charged and developed and are set in the second cycle, as shown in FIG. 2B, background fog occurs in a portion receiving the transfer corona ions to cause image degradation. The background fog and image degradation continuously occur not only in the first and second printing operations but until the end of operation of the transfer unit.
As described above, when a scheme for performing discharging and separating operations of a transfer medium using a grounded electrode having a sharp edge is used for obtaining a simple and low-cost separating scheme, a portion irradiated with the transfer corona is not sufficiently destaticized after a transfer operation. Therefore, even when the surface of the image bearing member is negatively charged in a later charging process, the surface of the image bearing member is not easily controlled to have a predetermined constant potential. For this reason, a potential difference between a charging potential and a developing bias cannot be sufficiently obtained, and background fog occurs. Therefore, a toner is always attracted to a region of the image bearing member surface except for portions covered with transfer media. Since the toner is removed by a cleaner, print cost is increased by an increase in consumption amount of the toner, an insufficient capacity of the cleaner is caused, and contamination in the machine occurs.
In addition, since a sufficiently high charging potential cannot be obtained between sheets, i.e., a sheet interval, due to an influence of the above transfer corona, image degradation such as background fog disadvantageously occurs as an electrostatic memory on a print image.